Don't store bunchspace as it may be updated

Pull request 1615 (already merged) fixed a problem with multiple time shifting of the same hits in the mixing module.
However, the Adjuster class that implemented this stored the bunchspace.  The Mixing Module allows the bunch space to be updated on run or lumi boundaries.  If this happened, the adjuster would still use the original value.

This simple pull request fixes this by passing in the bunch space as an argument, rather than storing it as a member of the Adjuster class.